The Gregorian chant was named after Pope Gregory I because:
It is a form of monophonic unaccompanied song in Latin, sung by the roman catholic church. It is a form of plain chant.
The Gregorian chant began in the middle ages in the Europe. It was a music of the catholic church and therefore used for ceremonial purposes.
This type of music was named after the pope because a dove came down from heaven and whispered the sacred music in his ear.
Therefore, the correct answer is option D.
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